The present invention relates generally to a dimmer circuit for controlling the RMS value of an AC voltage applied to a load. In particular, the present invention relates to a two wire dimmer circuit for use with reactive loads where damaging DC load current may be present. The present invention embodies correction means for decreasing damaging DC current flowing through the load and voltage compensating means for regulating the RMS value of the AC voltage applied to the load. A two wire low voltage dimming circuit without voltage compensating means but having improved dimming ability at low load currents is also disclosed.
The present invention has particular application to low voltage dimming systems wherein the load is a low voltage transformer. However, the present invention also has application to other types of loads, such as fluorsecent lighting systems.
Two wire dimming circuits are known. One conventional type of two wire dimming circuit comprises a triac and a double phase shift firing circuit operatively connected to the triac's gate terminal. The double phase shift firing circuit employs a series R-C circuit that is coupled across the triac, and a firing capacitor coupled to the R-C circuit by means of a potentiometer and to the gate terminal of the triac by means of a diac. This circuit corrects for damaging DC currents that are known to flow through reactive loads, such as the primary winding of a low voltage transformer, by adjusting the firing angle of the triac in selected half cycles of the waveform of the AC load voltage in the following manner. The DC component appears across the triac and hence also across what is known as the "leading capacitor". The leading capacitor is the capacitor in the before referenced series R-C circuit. Since the leading capacitor is connected to the firing capacitor through the potentiometer, the DC voltage across the leading capacitor is added to the voltage across the firing capacitor and the firing angle is altered to decrease the DC current.
While the aforementioned two wire dimming circuit is capable of solving the DC current problem that is known to exist in reactive loads, it exhibits poor voltage regulation. That is, it is not capable of maintaining the RMS value of the AC voltage applied to the load substantially constant with fluctuations in the AC supply voltage. It is known to modify such a two wire dimmer so that it exhibits good voltage regulation by replacing the leading capacitor with a diac. The modified circuit, however, cannot correct for the DC current problem once the leading capacitor has been removed.
Three wire dimming circuits are also known. In a three wire dimming circuit, two of the wires are connected directly to the AC supply voltage and the firing angle is determined from the voltage across the AC supply. Thus, the firing angle is not affected by DC currents that may flow through the load. It is known to those skilled in the art to incorporate a voltage regulating diac in the three wire dimmer's firing circuit. Such a three wire dimmer exhibits good voltage regulation and does not exhibit the DC current problem. Three wire dimmers, however, are undesirable because three wires (AC hot, AC neutral and load) must be run to each wall box, thus requiring additional installation cost.
The present invention overcomes the failings of the prior art by incorporating both a voltage regulation circuit and a DC current correction circuit in a two wire dimmer.